XI.] THE FRESH- WATER MUSSEL. Ill 



ft. A curved ventral border, opposite the dorsal. 



7. A wider anterior end. 



8. A narrower posterior end. 

 e. A right and left side. 



b. The mantle or pallium. 



a. A bilobed semitransparent membrane, one lobe 

 lining each valve of the shell. 



ft. The continuity of the two lobes on the dorsal 

 side of the animal; their separation along 

 most of its ventral side, where each forms a 

 thick yellowish free border. 



7. The union of the two pallial lobes, for a short 

 distance, towards the posterior part of their 

 ventral border. 



. The rudimentary dorsal and ventral siphons, 

 separated from one another at the point of 

 union 7. and each marked out by a part of 

 the mantle-edge covered by short hair-like 

 processes: the dorsal siphon completely closed 

 below and forming a narrow oval slit; the 

 ventral siphon open below and continuous with 

 the cleft between the ventral edges of the 

 mantle-lobes. 



. The branchial or pallial chamber: turn back 

 the ventral edge of that mantle-lobe from which 

 the shell has been removed: a chamber is thus 

 exposed into which the ventral siphon and 

 the cleft continuous with it, lead. 



